1. Field of the Invention
The instant invention relates to methods and apparatus for analyzing fluid inclusions and more particularly to such methods and apparatus in which a fluid inclusion formed in a material such as mineral, glass, semiconducting material, and the like is ruptured, and the gases released therefrom are analyzed.
2. Setting of the Invention
When natural minerals are formed, fluid present in the vicinity of the crystal may be trapped in microscopic defects known as fluid inclusions. These fluid inclusions may be ruptured to release the paleofluids contained therein in order to analyze the same. Such analysis can be used to determine information relating to the nature of the fluids present when the mineral was formed.
Analysis of fluid inclusions formed in sedimentary environments can yield information which is useful in the exploration for and production of oil and gas. For example, such studies can produce information relating to timing of hydrocarbon migration relative to rock formation, pathways of hydrocarbon migration, and the influence of hydrocarbons on rock formation.
Fluid inclusions in minerals may be formed at the time of mineral growth or they may form later when cracks in the mineral heal. Fluid inclusions formed at the time of initial mineral growth are referred to as primary inclusions, and those formed during healing of cracks in the already-formed mineral are known as secondary inclusions. Cracks which have formed and healed at different times in the mineral's past produce different generations of secondary inclusions which trap environmental fluids present at the time of healing of the crack.
Sometimes a mineral overgrowth which acts as a cement may form between and around previously-formed mineral growth. Environmental fluids may also be trapped in fluid inclusions formed in the cement.
In the past, a number of different techniques have been utilized to release fluids from the inclusions in minerals and in other substances, such as glass. Such techniques include crushing and drilling. In another technique, the material, for example, a naturally-occurring mineral, is heated thereby increasing the fluid pressure in the fluid inclusions until the same rupture thereby releasing the fluids. This technique is known in the art as thermal decrepitation. A related technique involves use of a laser beam. When the laser beam is directed toward an area of interest in the mineral, the fluids in the inclusions are heated thereby rupturing the inclusion and releasing the fluids.
In the past, mass spectrometers have been used to analyze gases released from fluid inclusions using one of the above-described prior art techniques. Typically, the gases are released by cutting or crushing the mineral or by thermal decrepitation. Whatever the technique for releasing the gas, the gases are released into a vacuum which is in communication with the mass spectrometer. When the fluids are released from the inclusions into the vacuum, the volatile liquids in the inclusions evaporate. The gases are provided directly to the mass spectrometer where they are ionized and thereafter qualitatively and/or quantitatively analyzed in the usual manner. The mass spectrometer may be used to analyze the chemistry of the gases and evaporated volatile liquids and/or to analyze the isotopic ratios of elements contained therein.
A problem exists with the various prior art methods for releasing fluids from inclusion in naturally-occurring minerals and the like. When utilizing techniques such as crushing, slicing, and drilling, invariably fluids from more than one inclusion are released substantially simultaneously. This is especially true when dealing with small inclusions. For example, fluid inclusions of interest in sedimentary minerals are typically less than 10 microns in diameter. Thus, the analysis undertaken, whether by mass spectroscopy or by other means, may be of a plurality of fluid inclusions. Moreover, the analyzed fluids may be from inclusions formed at vastly differing times, such as a mixture of primary and secondary inclusions or a mixture of different generations of secondary inclusions.
Also, the mineral sample to be analyzed may include a plurality of different minerals closely adjacent one another as well as mineral growth formed between and on the various minerals, all of which include fluid inclusions. When such a sample is crushed, sliced, or drilled, fluids from inclusions in different minerals or from one or more cements may be simultaneously released. Such techniques prevent accurate analysis of selected types of inclusions such as inclusions from a particular mineral or cement or such as only primary inclusions, only secondary inclusions, or only a selected generation of secondary inclusions.
Some theorize that when fluids are released from inclusions in naturally-occurring minerals by thermal decrepitation, single inclusions sequentially burst in response to increasing temperature. However, there is no known way to verify this. Data generated by mass spectroscopy analysis of gases, including evaporated liquids, released from fluid inclusions may be interpreted to mean that (a) only a single inclusion ruptured at a specified temperature or (b) groups of inclusions ruptured at a specified temperature.
Even if it could be verified that only a single inclusion at a time burst as temperature is increased, this technique does not permit selection of a single identified inclusion nor does it permit selection of one inclusion from among a class of characterized inclusions, such as primary inclusions, secondary inclusions, a selected generation of secondary inclusions, inclusions from a selected cement, etc. In other words, as the temperature increases, any of the inclusions in the sample being tested may rupture, and there exists no control over selection of a particular fluid inclusion or a fluid inclusion from among a particular class of inclusions to be ruptured.
The laser technique suffers from similar drawbacks. Typically, a selected area in a mineral sample is located using a microscope. Thereafter, a laser beam is shined through the microscope onto the sample and the heat generated thereby ruptures inclusions in the general area. Although the laser technique allows exercise of greater control over which inclusions are to be ruptured than thermal decrepitation of the entire sample, the heat produced by the laser beam is applied to a general area of the sample, and it is not possible to limit the technique to rupture only a single selected inclusion. Thus, the above-described drawbacks of the thermal decrepitation technique are also present when a laser is used to release gases, including evaporated volatile liquids from fluid inclusions. In addition, the laser heat can also release gases from volatile matter received in cracks in the sample or from adsorbed fluid in the sample. Therefore, at least some of the analyzed gas may be from sources other than fluid inclusions.
There exists a need for a method and apparatus for analyzing fluid inclusions in which a single identified fluid inclusion may be ruptured.
There exists a need for such a method and apparatus in which selected fluid inclusions from an identified class of inclusions may be selectively ruptured.
There exists a further need for such a method and apparatus in which a plurality of identified fluid inclusions may be individually and sequentially mechanically ruptured.